The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
This disclosure relates to compositions and methods for completing subterranean wells, in particular, fluid compositions and methods for completion operations during which the fluid compositions are pumped into a wellbore and make contact with subterranean rock formations.
In the course of completing oil and gas wells and the like, various types of fluids are circulated in the wellbore. These fluids include, but are not limited to, drilling fluids, spacer fluids, cement slurries and gravel-packing fluids. In addition, these fluids typically contain solid particles.
Cement slurries are usually incompatible with most drilling fluids. If the cement slurry and drilling fluid commingle, a highly viscous mass may form that can cause several problems. Cement slurry can channel through the viscous mass. Unacceptably high friction pressures can develop during the cement job. Plugging of the annulus can result in job failure. In all of these situations, zonal isolation may be compromised, and expensive remedial cementing may be required.
Consequently, intermediate fluids called preflushes are often pumped as buffers to prevent contact between cement slurries and drilling fluids. Preflushes can be chemical washes that contain no solids or spacer fluids that contain solids and can be mixed at various densities.
Chemical washes are preflushes with a density and a viscosity very close to that of water or oil. The simplest chemical wash is fresh water; however, for more efficient drilling-fluid thinning and dispersion, chemical washes that contain dispersants and surfactants are more commonly used.
Spacers are preflushes with carefully designed densities and rheological properties. Spacers are more complicated chemically than washes. Viscosifiers are necessary to suspend the solids and control the rheological properties, and usually comprise water-soluble polymers, clays or both. Other chemical components include dispersants, fluid-loss control agents, weighting agents and surfactants. A thorough discussion concerning the uses and compositions of preflushes may be found in the following publication. Daccord G, Guillot D and Nilsson F: “Mud Removal,” in Nelson E B and Guillot D (eds.): Well Cementing-2nd Edition, Houston: Schlumberger (2006) 183-187.
For optimal fluid displacement, the density of a spacer fluid should usually be higher than that of the drilling fluid and lower than that of the cement slurry. Furthermore, the viscosity of the spacer fluid is usually designed to be higher than the drilling fluid and lower than the cement slurry. The spacer fluid must remain stable throughout the cementing process (i.e., no free-fluid development and no sedimentation of solids). In addition, it may be necessary to control the fluid-loss rate.
As well depth increases, the formation temperature and pressure also increase. Consequently, to maintain well control and prevent invasion of formation fluids into the wellbore, the hydrostatic pressure exerted by the drilling fluid, spacer fluid and cement slurry must be higher than or equal to the formation pressure. In deep wells, it is often necessary to prepare fluids with densities between 2037 kg/m3 (17 lbm/gal) and 2756 kg/m3 (23 lbm/gal). In addition, the bottomhole temperature can exceed 260° C. (500° F.).
These conditions present challenges for those designing spacer fluids with optimal densities, rheological properties, stability and fluid-loss rates. Achieving high fluid densities typically requires the addition of heavy particles comprising barite, hematite, ilmenite and haussmanite. The solid volume fraction necessary to achieve high densities is also elevated. However, keeping the particles in suspension is difficult at high temperatures, possibly leading to spacer instability. It would be therefore desirable to provide means by which spacer-fluid rheological properties, stability and fluid-loss control may be better controlled at elevated temperatures.